1. Field of the Invention
This invention relates to an image reading apparatus for digitally reading image light from an original by a sensor through a slit.
2. Related Background Art
The relative alignment (such as positioning in a sub-scanning direction and inclination) of a slit and a line sensor in an image reading apparatus has heretofore been done during the assemblage of the image reading apparatus. That is, it has been highly accurately done in a well-equipped factory by the use of a jig exclusively for use for optical adjustment.
However, when installing and using an image reading apparatus in the market, there arises a problem that distortion is created in the entire apparatus due to the vibration or fall of the apparatus by transportation or the like or to the planarity or the like of the installation plane and the relative positional relationship between a slit and a line sensor shifts to thereby reduce image reading accuracy.
Particularly, in an apparatus as shown in Japanese Laid-Open Patent Application No. 3-179868 previously proposed by the applicant wherein a light beam from a colored image is separated into three color lights such as R (red), G (green) and B (blue) by color resolving means using a diffraction grating and the colored image is read on the basis of said three color lights, the positional accuracy of the light beam incident on the diffraction grating has been very severe and very high accuracy has been required of the position of the slit.
So, the applicant has previously proposed in U.S. Application Ser. No. 863782 , now U.S. Pat. No. 5,283,698 an apparatus in which a scanning optical system is provided with means for detecting the relative position between a slit for forming an image on a line sensor provided in the scanning optical system.
In this apparatus, however, there has been a problem that the detection of the relative positional relationship between the slit and the line sensor is only effected at one location such as the home position corresponding to the back side of the shading correcting plate of an original supporting table and the image of the whole glass of the original supporting table cannot the ensured.